The invention relates generally to the field of flat plate solar collector devices used to absorb and transfer solar energy into a collection fluid. More particularly, the invention relates to such devices which are evacuated to increase efficiency and which incorporate a combination of coatings and internal components to enhance reception of incoming solar energy and to reduce outbound radiation losses from the device.
The invention was developed to provide high operating temperatures at high efficiencies from solar energy while avoiding reflecting, tracking, focusing, or vacuum tube type collectors which have traditionally been used for such purposes. Further, the invention was developed to provide an easily manufacturable evacuated flat plate solar energy collector. Previous attempts at delivering an evacuated flat plate collector have provided structures sufficient to resist the forces applied by the atmosphere to an evacuated envelope, but were very difficult and costly to manufacture. More importantly, the previous designs do not effectively deal with thermal radiation losses. The present invention uses a true flat plate absorber, flat gluing and is non-focusing. Focusing severely limits the capture of available solar resources and does not work unless the sunlight is clear, direct and bright. The present invention operates well in diffuse sunlight as well as direct sunlight.
It is well known that the conventional flat plate design is most reliable in its standard operating range and derives the highest operating efficiencies of all the various types of solar collectors. It also does not require direct bright sunlight to operate. Drawbacks associated with traditional flat plate design are that efficiency is a decreasing second order function dominated by convection heat transfer losses, conventional materials of construction cannot survive operational temperatures above 175 degrees C., little attention has been given to thermal radiation heat transfer, and efficient operation occurs only within a relatively small temperature differential above ambient, typically about a 25 to 40 degrees C. range. These limitations are due to the heat losses from the absorber plate due to convection, conduction and thermal infrared emissions, commonly called radiation. Attempts at increasing the operating range of flat plate collectors have resulted in marginal increases in temperature, but trade off efficiency and drive up material and manufacturing costs. Evacuated tubes have been shown to maintain relatively higher temperatures, but fall short of the temperature and efficiency goals needed to produce 6 to 7 bar steam which is required by industry. Focusing, also known as concentrating or reflecting, designs have been more successful from both a cost and efficiency standpoint, but require unoccluded direct sunlight.
It is an object of this invention to produce a flat plate collector comprising a sealed casing comprised of the combination of a flat primary glazing and a rear housing, the rear housing configured to comprise a number of linear cells semi-circular in cross-section, each cell containing a fin-tube collector with collection fluid to absorb the collected heat energy and transport it to an external use, where the amount of inbound solar radiation is optimized and the amount of outbound radiation losses from the device are reduced, thereby providing for a flat plate solar collector capable of high operating temperatures and high efficiency. It is a further object to produce such a device which is capable of delivering up to 30 bar steam with a 50% conversion of sunlight into 232 degree C. fin-tube temperature. It is a further object to provide such a device which incorporates a combination of anti-reflective coatings, low emissivity coatings and internal radiation shields to maximize efficiency. It is a further object to provide such a device where the internal temperature of the device is regulated by selectively evacuating and reintroducing gas into the interior of the casing. It is a further object to provide such a device with a secondary glazing either external or interior to the primary glazing.